Micro-lenses enhanced surfaces can be formed on a variety of surfaces and can be made using any number of materials and processes. A common form of micro-lens enhanced surface is the lenticular lens sheet. The lenticular lens sheet comprises a substrate or web with a top surface having a side-by-side array of substantially parallel refractive optical ridges and with a bottom surface that is generally flat.
In application, the lenticular lenses of the lenticular lens sheet typically receive light that passes from the direction of the flat surface toward the ridges and direct such light in a way that sends different portions of the light entering each lenticular lens to different portions of a viewing area in front of the lenticular lens. This light distribution function is commonly used to enhance viewing angles in rear projection television systems.
The light distribution function is also commonly used in conjunction with specially printed interlaced images to achieve various visual effects including motion effects and depth effects. See for example, commonly-assigned U.S. Pat. No. 5,715,383 (Schindler et al.).
The interlaced images used with lenticular lenses typically comprise a substrate having parallel strips of recorded image information, the image bearing substrate being arranged to cooperate with the lenticular lenses, typically by affixing or otherwise positioning the image bearing substrate proximate to or against the flat surface of the lenticular lenses.
The parallel strips of recorded information represent image information from at least two different images. The interlaced image is typically then affixed to the flat surface so as to be viewed through the lenticular lens array. The image information used in forming the interlaced images is determined so that the lenticular lenses will direct light from different images toward different portions of a viewing space proximate to the viewing area so that a viewer viewing the image modulated light from a first portion of the viewing space will see different image information than a viewer viewing the resultant image from another portion of the viewing space.
While such images are popular with consumers it has proven difficult, in practice, to provide a high quality lenticular lens enhanced article. This is because it is typically quite difficult to fabricate lenticular lenses that have uniformly desirable optical properties.
In some cases, the difficulty in forming such lenticular lens enhanced articles arises because the manufacture of lenticular lens enhanced articles requires engraving a master relief pattern and then replicating lenticular lens sheets from the master. A number of conventional manufacturing methods have been developed to produce lenticular lens enhanced articles with the useful optical characteristics. These include machining, platen press, injection or compression molding, embossment, extrusion, and casting. The materials used to form the lenticular lenses for such articles include a variety of clear optical materials such as glass and many types of plastics. Each of these prior art methods suffers inherent problems which render them ineffective for the high-volume production of lenticular lens enhanced articles or other forms of micro-lens enhanced articles.
For example, machining can be used to directly manufacture coarse, one-of-a-kind large lenticular lens enhanced articles such as in thick plastic sheets. Milling machines or lathes can be used with a diamond tip tool having a pre-determined radius. However, machining is a slow and costly process. This method for manufacturing lenticular enhanced surfaces is not well-suited to volume production.
In another example, a platen press can be used to stamp or emboss an engraved relief pattern into a thermoset material. The temperature of the thermoset material is raised to soften the material so that it conforms to the engraved surface. The temperature of the material is reduced to harden the material such that it retains the relief pattern when removed from the platen press. Like machining, this method is slow and expensive. Furthermore, the sheet size is limited. This method is not suited for high-volume production or for producing a continuous length product. Similar problems apply to injection or compression techniques for manufacturing molded lenticular lens enhanced articles.
In still another example of a method for manufacturing lenticular lens enhanced articles, extrusion embossment in continuous length roll form is used. Typically, these systems utilize an engraved roller with a thread-like screw pitch to the relief pattern. While such techniques enable relatively high-volume production, the quality and definition of extrusion relief patterns are generally inferior to patterns obtainable by platen or ultra-violet casting methods.
Extrusion techniques are also commonly used to help manufacture lenticular lens enhanced articles in relatively high-volumes. However, such techniques have difficulty maintaining the absolute parallelism of the lenticular rows. Due to the elastic nature of the molten plastic material and the internal stresses imparted by the embossing roller, the sheet has a tendency to change from its impressed shape prior to being fully set. Additionally, extrusion lenticular sheets can streak due to condensation, adding to the dimensional distortion and migration of the lenticular surface. These dimensional distortions create optical defects in the lenticular lenses that result in serious distortions and degradations in the perceived image. Migration is the tendency of the extruded plastic to move in a direction perpendicular to the direction of lenticulation during the extrusion process. Migration can also create dimensional distortion.
The optical quality of extruded lenticular lenses also suffers from the influences of the polymers from which they are formed. Some extrusion systems attempt to control this problem by curtain coating the polymers to a pre-extruded non-lenticulated web sheet requiring a binder coating to produce the multi-layered ply-sheet. Curtain coating is a process in which a flow of liquid plastic is set by a chill roller. This does not control the migration problem and adds defects such as bubbles, separation of surfaces, and diffusion of images, thus reducing the optical quality of the lenticular sheet.
Due to fabrication problems such as these, common practice has been to attempt to modify the process of generating and printing the interlaced image in various ways in order to conform the interlaced image to actual measured optical properties of the lenticular lenses. However, even where this is done, difficulties arise in meeting the challenge of assembling the lenticular lens array sheets to the printed interlaced image in proper registration. Typically, these challenges are met by labor intensive operations.
Some of these assembly issues have been addressed by a photographic technique using a composite sheet having a back surface coated with a photosensitive emulsion. The stereoscopic images are obtained as multiple exposures of the photosensitive emulsion through a lenticular screen. The composite sheet has a layer of cured thermosetting polymer on one surface of a base polymer film. The patterned lenticular relief is imposed upon the thermoset layer by curing the thermosetting resin while it is wrapped around a molding surface. The technique requires that it be used only with continuous roll transparent films. The disadvantage of this approach is that only special dedicated equipment can produce overall full-width continuous roll transparent films having lenticular lenses on at least one surface. This of course is a complex and expensive operation that further requires a separate fixing step during which the exposed photosensitive material is converted into an image having a generally fixed appearance.
In still another alternative, the challenges of assembly are addressed by directly printing the interlaced image onto the flat surface of the lenticular sheet. This too is challenging and time consuming for conventional printing operations because of needs for greater precision, tight registration of the interlaced image to the lens, and correction for press induced distortion of the lens, requiring special printing techniques, custom equipment, and setup.
U.S. Pat. No. 5,330,799 (Sandor et al.) describes a method and apparatus for producing autostereograms using ultraviolet radiation-curable thermosetting polymers. A stereoscopic image is printed upon a plastic or paper sheet, which is fed directly onto a surface having an inverse lenticular pattern relief. As the sheet is fed onto the surface, a flow of ultraviolet-curable thermosetting polymer resin is directed at the surface. Ultraviolet radiation is directed at the polymer layer, curing the polymer and forming a lenticular array on the front surface of the polymer layer using a lenticular master consisting of inverse lenticular lenses. During this process, the sheet carrying the stereoscopic image is bonded to the back surface of the polymer lenticular layer in precise registration with the lenticular array. Only those parts of the printed image requiring micro-lenses are treated in this fashion. Since the printed image and the lenticular master are both pre-made, this invention still faces all the complications associated with alignment and registration.
In U.S. Pat. No. 5,460,679 (Abdel-Kader) describes a method for forming micro-lenses on a previously offset-printed image using screen-printing. An optic screen of finely spaced lines is formed as a cured emulsion on a mesh silk-screen. A clear gel is extruded through the mesh screen onto the front side of a clear plastic sheet, creating an array of lenses. An image is previously printed on the back side of the plastic sheet using an offset printer. An optic grid of lines is superimposed in the image. The optic grid has a relationship with the lenses to create special effects such as depth enhancement.
U.S. Pat. No. 6,546,872 (Huffer et al.) provides a method for making raised resin profile ridges using energy-curable inks and energy-curable coatings, for example, UV-curable inks and coatings, having differential surface tensions or different surface energies. The steps of the method include: (a) providing a transparent substrate sheet having a front and a back; (b) printing an array of substantially parallel lines in at least one energy-curable ink on the front of the sheet; (c) applying at least one energy-curable coating over the array printed in energy-curable ink, the ink and coating being chosen so that sufficient repulsion is created on contact between the ink and the coating to form an aligned series of contiguous beads of coating material before curing takes over to ensure the formation of a raised ridge structure over the image printed in energy-curable ink; and (d) curing to produce a stable pattern of raised resin profile ridges that follows the pattern of printed lines. Notably, the image and the lenticular lens arrangement are placed on opposite sides of the transparent substrate.
It will be appreciated that the approach of U.S. Pat. No. 6,546,872 creates difficulties when combined with conventional image printing in that a substrate is called upon to absorb both the inks or dyes used to form an image and the additional inks or dyes used to form the parallel lines of repulsive material. This can create difficulties where the printed image is printed using inks that inherently have some degree of repulsion, or where the inks or varnishes used to create the lines of repulsion interact with the inks used to form the image. Further there is a danger of oversaturating the substrate with inks or varnishes. Thus, the use of such techniques with particular images must be carefully considered and the results for any given interaction are not necessarily predictable.
It will also be appreciated that the aforementioned techniques generally assume that the lenticular sheet is co-extensive with the entire area of the image. However, the needs for printing applications are very different. For example, cost, weight, or other factors may cause a publisher to wish to avoid publishing entire pages of documents in lenticular form. Thus, for example, it may be useful to provide a three-dimensional or motion picture area as a part of a sheet or page of a book, it is much less desirable to do so where such an image will occupy an entire page.
Thus, there remains a need for a simple, flexible and efficient method to create useful lenticular lens arrays that are correctly registered to a printed image. There is a further need for greater variety in the form, distribution and arrangement of micro-lenses of other types that can be used with co-designed printed images to provide micro-lens enhanced articles that provide particular visual effects and that can be formed in a reliable fashion using generally available commercial resources.